Remove old prepre method in example user migration. It was setting ->created=0 instead of current time.

Add dedupe() method for a field mapping. This assures that the value is unique, even if source data contains duplicates. Commonly used for usernames. Uniqueness is currently achieved by appending a number (e.g. example1). In the future we can expand to custom replacement patterns such as email address: sally1@example.com. Dedupe can be used on any mapping.

backport fix. Don't use URL to get XML file since thats not too reliable working through drush. Use file path instead

#753284 by mikeryan: Basic implementation of sourceOfRecord==DESTINATION handling, backported from D7